These skyboxes are available to Portal 2 and mods utilizing its content.
These skies are HDR capable. Go to Map Properties and change "SkyBox Texture Name"
to whatever sky you want. An env_fog_controller is also recommended.

However, this list is out dated and after the Wildfire update, using a light_directional will cause problematic shadows. GCFScape can be used to view the latest skies available. They are located in the pak01_dir.vpk file under C:\Program Files (x86)\Steam\steamapps\common\Counter-Strike Global Offensive\csgo\pak01_dir.vpk and in GFCScape under root/materials/skybox/